Tackified acrylic copolymers have long been used as pressure-sensitive. Such tackified copolymers have been prepared by adding a tackifying petroleum-based hydrocarbon resin or natural resin with an acrylic copolymer. While the resulting adhesive is often satisfactory to certain properties, such as tackiness and peel strength on substrates such as polyester or stainless steel, certain properties are found to be deficient, particularly the holding power on these substrates. Previously, increasing the peel strength of tackified acrylic copolymers to a value greater than 2.0 pounds per inch often caused diminishing shear adhesion fail temperatures (SAFT) and holding power. It is desirable to maintain high holding power and SAFT, particularly when the pressure-sensitive adhesive is to be used to prepare an adhesive tape.
The prior art describes attempts to prepare satisfactory tackified acrylic copolymers. For example, Japanese Patent J-59213783 teaches the preparation of a hot-melt pressure-sensitive adhesive by first heating a tackifying resin having a softening point between 60.degree. and 200.degree. C. to above its melting point, and adding to the hot melt a polymerization mixture of an alkyl (meth)acrylate, a functional monomer such as acrylic acid, and a radical initiator over a period of hours, with stirring, to form a pale yellow, transparent, solid hot-melt adhesive.
Japanese Patent J-59227967 discloses a hot-melt polymerization of an alkyl (meth)acrylate main monomer and a copolymerizable functional monomer including (meth)acrylic acid, maleic anhydride, maleic acid, and the like, where a surfactant is present with the resin during the polymerization to form a solid mass, hot-melt adhesive. In Japanese Patent 53074041, a polymerized powdery toner product is obtained by dissolving a binder resin such as a vinyl resin, acetal resin, epoxy resin or the like in a polymerizable liquid monomer such as styrene, vinyl toluene, (meth)acrylic acid or its ester, and the like, and polymerizing the monomers in bulk.
In Japanese Patent J-51125472, a petroleum resin emulsion is obtained by polymerizing vinyl monomers in the presence of petroleum resins having softening points of from 40.degree. to 160.degree. C., an average molecular weight of 300 to 3000, and an acid value and saponification value of less than 1. The monomers include for example, alkyl (meth)acrylates, vinyl acetates and vinyl chlorides, styrene, acrylonitrile, and acrylic acid. The emulsified mixture is then reacted in an emulsion polymerization reaction to form a shelf-stable emulsion adhesive. The resin emulsion produced in described as having fine particle sizes and ample stability, and when cured, the films produced have excellent water resistance and gloss.
Numerous approaches have been used to produce resin emulsions. One approach is to dissolve the resin in a hydrocarbon solvent, combine the resin solution and water to form an emulsion, and strip off the solvent. Invariably some residual hydrocarbon solvent remains in the finished emulsion, which is undesirable in certain applications. This has led to the development of solvent-free dispersions (see U.S. Pat. No. 2,809,948) and emulsions (see U.S. Pat. No. 3,377,298) of petroleum resins. In both of these formulations, ionic emulsifiers have been utilized; in the former a mixture of cationic and non-ionic surface active agents is used to achieve a resin emulsion; and, in the latter an ionic surfactant is used in combination with an aqueous gel of a swelling earth to produce an emulsion paste of a petroleum resin.
As mentioned above, there has generally been a decrease in shear properties associated with an increase of peel strength and tackiness. There is a need to improve peel strength of pressure-sensitive adhesives while still maintaining a high SAFT.
My copending application Ser. No. 360,507, filed Jun. 2, 1989, for "Internal Resin-Tackified Acrylics Polymers" (P-1413) deals with the problem somewhat but further improvement has been found to be desirable, particularly in improving the holding power at higher temperatures (SAFT).
Accordingly, it is an object of this invention to prepare a resin-tackified acrylic copolymer which has an improved peel strength and SAFT for application as pressure-sensitive adhesives.
It is a further object to provide a resin-tackified acrylic copolymer, wherein the tackifying resin is dissolved in the acrylic monomer solution with a crosslinkable monomer prior to the polymerization reaction to improve the peel strength and adhesion properties.
It is still further objective of this invention to cause additional improvement in the SAFT by incorporating into the monomer polymerization mixture a crosslinkable comonomer.
It is a still further object of this invention to provide pressure-sensitive adhesives useful to prepare tapes and useful laminating adhesives.